Does a sinus infection cause hair loss?

See one of the contributing factors to Sinus is cold environment.

This means less warmth in your head, which means less blood to your head, scalp.

Which means less blood for your hair to grow. Also the nerves in the head are as tiny and thin and finer than size of hair. So yes, blocked sinus means less blood flow to your head.

Off course, it is just one of the factors. Also all those who have Sinusitis do not have hairloss.

Yes, there is a connection but only you will be able to know if its a major, minor or just contributing factor.

Its like all those who eat ice cream dont catch cold, so you have to know if Sinus is causing hairfall in your case. Are you sure nothing else is causing hair loss to you? Then yes, sinus can be a reason.

If it is, then get your sinus cured, else it could because of other reasons.